Gear driven compressor with vent compressor

ABSTRACT

A two-stage compressor includes an auxiliary compressor that reduces the pressure of the gear section. The auxiliary compressor intake communicates with the gear section and the discharge of the compressor communicates with the inlet to the second stage of the compressor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to the art of gear driven compressors. In particular, the invention is directed to a method and system for reducing drive train losses in gear driven compressors and for reducing foaming by dissolved gases from the oil.

[0003] 2. Description of Related Art

[0004] Drive train losses in a gear driven compressor can be significant. These losses can be due to several things, but windage appears to be a significant factor that adds to the inefficiency of a gear driven compressor. The rapidly rotating gears must overcome the windage which is generally defined as the resistance to movement of the gears through the gas in the compressor. The windage is directly proportional to the density of the gas.

[0005] Conventionally, gear driven compressor designers have vented the gear chamber to the inlet to the compressor to reduce the pressure in the gear chamber, which reduces the density of the gas. However, leaks develop which reduce the flow of this venting. For example, a leak typically develops at a seal between the gear chamber and an adjacent compressor chamber which reduces the flow through the vent. Leaks through the seal between the compressor impellers and the gear cavity tend to increase the gas pressure in the gear cavity and a sufficiently large flow passage must be provided between the gear cavity and the compressor inlet to insure that the pressure in the gear cavity is nearly as low as in the compressor. Also, this gas which is vented into the main inlet to the compressor requires energy to be compressed through both stages of the compressor. The first and second stages of the compressor must expend energy to compress this gas to the outlet pressure.

[0006] One conventional compressor provides a small auxiliary compressor to compress the gas from the gear chamber into the main inlet of the compressor. Therefore, the first stage of the compressor has to expend less energy to bring that gas up to the pressure of the gas entering the second stage.

[0007] Additional problems are experienced upon startup of a conventional compressor. Normally, a compressor is started from atmospheric conditions and the pressure in the entire compressor housing can be quite high. This high pressure on the lubricant for the gears causes a significant amount of gas to dissolve into the lubricant. When the compressor is started and the pressure is reduced, the gas foams out of the oil and interferes with the starting process. As a result of this problem, it is common practice to heat the lubricant prior to startup to boil the gas out of the lubricant and to reduce the foam during startup.

[0008] Lastly, high pressures that are conventionally experienced in gear driven compressors cause leaks to develop through a seal that separates the gear chamber from the atmosphere. Thus, lubricant and gases escape into the atmosphere and are wasted.

SUMMARY OF THE INVENTION

[0009] An aspect of an exemplary embodiment of the invention improves the efficiency of a gear driven compressor by providing an auxiliary compressor that draws gases out of the gear chamber of a two-stage gear driven compressor, compresses the gas and supplies the compressed gas to the inlet of the second stage of the compressor. The auxiliary compressor reduces the density of the gas in the gear chamber to approximately one quarter that of the gas at the inlet to the compressor. The energy expended in running the auxiliary compressor is more than compensated for by the amount of energy saved by the reduction in the windage loss.

[0010] While it may have been known to compress the gas in a gear chamber to the pressure of the gas at a main inlet of a compressor, the invention reduces the amount of energy expended by the second stage to bring the pressure of that gas up to the discharge pressure by providing an auxiliary compressor which compresses the gas from the gear chamber to the pressure of the inlet to the second stage. Thereby, removing the requirement for the first stage compressor to expend energy. This savings in energy in the first stage is more than made up by the amount of energy expended by the auxiliary compressor and the energy savings in further reducing the pressure in the gear chamber.

[0011] Another aspect of an exemplary embodiment of the invention improves the startup conditions of the compressor. The auxiliary compressor may also be used prior to startup to significantly reduce the pressure in the gear chamber and to release any gases dissolved in the lubricant. This obviates the need to heat the lubricant prior to startup, and eliminates the cost of heating the lubricant. The auxiliary compressor may also be operated upon compressor shut down to reduce gases dissolved in the lubricant during shut down that may reduce the viscosity of the lubricant.

[0012] The reduction of the pressure in the gear chamber by the auxiliary compressor also substantially eliminates any leakage of gas and lubricant into the atmosphere through the seals between the compressor and the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Exemplary embodiments of this invention will be described in detail, with reference to the following figures, wherein:

[0014]FIG. 1 is a cross-sectional view of an exemplary embodiment of a two-stage compressor in accordance with the invention.

[0015] These and other features and advantages of this invention are described in or are apparent from the following detailed description of exemplary embodiments.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

[0016]FIG. 1 shows a cross-sectional view of an exemplary embodiment of a two-stage compressor 100 in accordance with the invention. The compressor 100 includes a first stage section 102, a second stage section 104 and a gear section 106 and a compressor housing 108. A motor (not shown) is coupled to a first drive shaft 110. The first drive shaft 110 is supported in the compressor housing 108 by a bearing 112. The compressor housing 108 also includes a seal 114 that seals the exterior of the compressor housing 108 from the gear section 106. A gear 116 is mounted on the first drive shaft 110 inside the gear section 106. The gear 116 drives cluster pinion gears 118 which are connected to gears 120 and which, in turn, drive another pinion gear 122.

[0017] Pinion gear 122 is attached to a second drive shaft 124 which extends into both the first stage section 102 and the second stage section 104. Two centrifugal impellers 126 and 128 are mounted on the end of the second drive shaft 124. The gear system increases the rotational speed of the first drive shaft 110 to a much higher rotational speed on the second drive shaft 124.

[0018] Gas enters a first inlet 130 and flows through impeller 126 into a first discharge section 132. A pipe (not shown) carries the gas from the first discharge section 132 to the second stage inlet 134. The gas flows through impeller 128 and into a second discharge section 138 and subsequently out a discharge pipe (not shown). The refrigerant may be compressed from an inlet pressure of approximately 83 psia to a discharge pressure of 210 psia. Also, the pressure in the first discharge section 132 and the second stage inlet 134 may be approximately 132 psia. However, it should be recognized that a two stage compressor is not required to have any specific or approximate pressures in order to incorporate the features of the invention and that the approximate pressure set forth herein are merely for example purposes.

[0019] Since the pressure is higher in the second stage inlet 134 than at the first inlet 130, if the gear cavity 140 is vented to the first inlet 130, then there is a leakage flow of the gas from the second stage inlet 134 at the inlet of the impeller 128 and the gear cavity 140. This leakage is minimized by a close fitting shaft seal 142 between the impeller 128 and the support bearing 144. Compressor 136 compresses and discharges the gas leaking from the second stage inlet 134 into the gear cavity 140 back into the second stage inlet 134.

[0020] The two-stage compressor 100 includes an auxiliary compressor 136 that draws gas from the gear cavity 106, compresses the gas and discharges the compressed gas into the second stage inlet 134. In this manner, the density of the gas in the gear cavity 106 can be substantially reduced to, for example, approximately one quarter the density of the gas at the first inlet 130. Thereby advantageously significantly reducing the windage of the lubricant in the gear cavity 106. For example, in a typical case, the total horsepower required to drive the two-stage compressor without the auxiliary compressor might be 706 horsepower. Approximately fourteen horsepower is required to overcome the windage created by the rapidly rotating gears. By adding the auxiliary compressor 136, the horsepower to overcome the windage can be reduced to approximately three horsepower. The horsepower required by such a small auxiliary compressor is typically only 1 horsepower. Thus, the horsepower required to overcome windage is reduced by approximately eleven horsepower and the net savings in horsepower is approximately 10 horsepower.

[0021] The auxiliary compressor may be a small hermetic compressor with a totally enclosed motor, a small reciprocating compressor, a small scroll compressor or the like. In general, any small compressor that is capable of reducing the pressure in the gear section is acceptable to the invention.

[0022] The auxiliary compressor may be run prior to startup of the main compressor to reduce the pressure in the gear section. Reducing the pressure in the gear section allows any gases dissolved in the lubricant to be released prior to starting of the primary compressor. As a result, the present invention minimizes foaming typically experienced by release of the gas during startup thereby avoiding the adverse affects of foaming on the compressor startup process. Thus, the auxiliary compressor obviates the need for a heater prior to startup which is typically required by conventional compressors to reduce foaming. Also, any gases, such as a refrigerant, dissolved in the lubricant reduce the viscosity and, therefore, the lubricating effectiveness of the lubricant. Refrigerants can significantly reduce the viscosity of the lubricant in a compressor. For example, for R-22 dissolved in oil at 120 degrees and 80 psia the kinematic viscosity is approximately 4.8 centistokes, while at 120 degrees and 20 psia the kinematic viscosity is approximately 14 centistokes. Thus, it is desirable to reduce the amount of gases or refrigerants in the lubricant solution to a minimum.

[0023] Additionally, the auxiliary compressor may be run during main compressor shut down to reduce the opportunity for oil to leak into the atmosphere when the saturation pressure of the oil is higher than atmospheric pressure. For example, at 85 degrees fahrenheit, the saturation pressure of R-22 is 155 psig. This pressure may cause a leak of oil to the atmosphere through seal 124. To reduce and substantially eliminate this risk, the compressor 136 may reduce the pressure in the gear cavity 140 to atmospheric pressure. Reducing the pressure in the gear cavity 140 in this manner also reduces the amount of refrigerant dissolved in the oil, so that the compressor 100 may be started more quickly.

[0024] Preferably, the auxiliary compressor reduces the pressure in the gear section to approximately atmospheric pressure to reduce or substantially eliminate any pressure differential across any seal between the gear section and the atmosphere surrounding the compressor. By reducing the pressure differential, the potential for these seals to leak is substantially reduced or eliminated.

[0025] While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations are apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention as set forth above are intended to be illustrative and not limiting. Various changes may be made without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A two-stage compressor comprising: a compressor housing; a first stage compressor section within the compressor housing having a first inlet and a first discharge; a second stage compressor section within the compressor housing having a second inlet and a second discharge, wherein the first discharge section is in communication with the second inlet; a gear section within the compressor housing; and an auxiliary compressor in communication with the gear section and the second inlet.
 2. The compressor of claim 1, wherein the compressor is a centrifugal compressor.
 3. The compressor of claim 1, wherein the compressor is a refrigerant compressor.
 4. The compressor of claim 1, wherein the auxiliary compressor is a hermetic compressor.
 5. The compressor of claim 1, wherein the auxiliary compressor is a reciprocating compressor.
 6. The compressor of claim 1, wherein the auxiliary compressor is a scroll compressor.
 7. A method of starting a two-stage compressor having a compressor housing, a first stage compressor within the compressor housing having a first inlet and a first discharge section, a second stage compressor within the compressor housing having a second inlet and a second discharge section, wherein the first discharge section is in communication with the second inlet, a gear section within the compressor housing and an auxiliary compressor in communication with the gear section and the second inlet, the method comprising the steps of: operating the auxiliary compressor for a period of time; and starting the two-stage compressor.
 8. A method of operating a two-stage compressor having a compressor housing, a first stage compressor within the compressor housing having a first inlet and a first discharge section, a second stage compressor within the compressor housing having a second inlet and a second discharge section, wherein the first discharge section is in communication with the second inlet, a gear section within the compressor housing and an auxiliary compressor in communication with the gear section and the second inlet, the method comprising the step of operating the auxiliary compressor to reduce the pressure within the gear section.
 9. The method of claim 8, wherein the auxiliary compressor reduces the pressure within the gear section to approximately atmospheric pressure. 